1. Field of the Invention
The present invention is directed to a system and lead for monitoring blood oxygen concentration in a patient's venous system.
2. Background
Blood oxygen concentration is a direct indicator of the physiological status and need of a patient. Other indicators include various cardiac timing signals, blood temperature, respiratory rate and blood pressure measured within the chambers of the heart, blood oxygen saturation, as well as physical rate and acceleration. Physiologically responsive pacemakers and cardioverter/defibrillators, both referred to as cardiac stimulation devices, utilize one or more of such indicators as a control parameter(s) to vary cardiac rate in the process of optimizing a patient's cardiac performance to achieve the correct metabolic rate for providing a high level of patient well being. As is recognized in the art, cardiac stimulation devices provide patients having a chronically malfunctioning heart, with selective cardiac therapy in the form typically, of cardiac stimulation pulses in order to maintain proper metabolic rate. Unlike a healthy heart, a malfunctioning heart normally does not adequately respond to physical activity, exercise, stress or emotion, posture changes and sleep conditions. Normal cardiac reaction to physiological need manifests in adjustment of cardiac stroke volume and heart rate with the natural adjustment of such parameters being responsive to immediate need.
To remedy the deficiencies of a malfunctioning heart, cardiac stimulation devices have been widely used to control heart rate and thus metabolic rate. Early devices were not rate responsive and provided a pacing regimen that stimulated the heart at a constant rate under all conditions. The drawback of such devices was that under many conditions they did not provide the proper metabolic rate for patient need. For example, under sleep conditions where the required heart rate is low due to minimal physiological need, the device would pace the heart at too high a rate than required. On the other hand, a constant pacing rate did not adequately respond to a patient's metabolic need under high exercise and stress conditions.
Upon the introduction of rate responsive stimulation devices, a more rapid response to satisfying a patient's physiological need was made available. The intent of such devices was to adjust the patient's cardiac rate on a continual basis for a wide variety of exercise and stress conditions encountered by a patient. A very straightforward technique for controlling cardiac pacing in rate adaptive dual chamber devices is by the use of atrial tracking pacing, especially when the patient has partial or complete AV block. This technique however lacks effectiveness when the patient suffers from atrial fibrillation or sinus bradycardia.
To address this situation, other techniques utilizing the aforementioned indicators of physiological need have been proposed. For example, the use of venous blood temperature has been described by Cook et al., in U.S. Pat. No. 4,436,092. The shortcomings of this device are, lack of rapid response, coarseness of measurement and susceptibility to non-physiological influences such as the ingestion of cold liquids and fever. A second approach is the use of blood pressure at different locations in the heart. One such device utilizing the blood pressure in the atrium, was described by Cohen in U.S. Pat. No. 3,358,690. However, patients with sinus bradycardia and atrial fibrillation are not amenable to this approach. Other devices utilizing blood pressure have been described by Koning et al. in U.S. Pat. No. 4,566,456 and by Scroeppel in U.S. Pat. No. 4,600,017. These devices however, fall short if the patient suffers from sinus bradycardia or atrial fibrillation. Activity sensing, rate responsive devices as described for example by Anderson et al., in U.S. Pat. No. 4,428,378 and by Thornander et al., in U.S. Pat. No. 4,712,555 provide fast response times and good reliability but often are affected by non activity events. For example, if the patient is riding in a vehicle over a bumpy road, the resulting body movement will be erroneously interpreted as physical activity inducing unnecessary pacing rate increases.
Still, another class of devices utilizes blood chemistry as physiologic indicators. The use of blood oxygen saturation has been described by Wirtzfeld et al. in U.S. Pat. Nos. 4,202,339 and 4,399,820; by Bornzin in U.S. Pat. No. 4,467,807; by Cohen in U.S. Pat. No. 4,815,469; by Thompson in U.S. Pat. No., 5,342,406; by Thacker in U.S. Pat. No. 5,438,987; and by Mortazavi in U.S. Pat. Nos. 5,040,538 and 5,411,532. The devices taught by Mortazavi include a light-emitting diode (LED) positioned within a chamber of the heart and arranged such that light emitted from the LED is directed at the blood which then reflects the light to an adjacent light detecting sensor in the form of a phototransistor. Upon delivery of a sensing pulse, the LED commences to emit light and the reflected light is produced in an intensity proportional to the oxygen concentration. Circuitry associated with the phototransistor integrates the voltage proportional to the light's intensity and when the integrated voltage reaches a “prescribed value”, current is then shunted away from the LED and the integration process is terminated. The time interval from delivery of the sensing pulse to reaching the “prescribed value” is a measure of the degree of oxygen saturation in the blood. The blood cells change from a blue to a red hue when the myoglobin in the blood cells is saturated with oxygen. This sensing method does not indicate accurately the percentage of oxygen in the blood below the saturation point of the myoglobin. The oxygen sensor circuit includes a diode in series circuit arrangement with a stimulation lead conductor. Pacing pulses are generated having a first polarity, while oxygen sensing pulses are generated having a second polarity opposite to that of the first polarity. Accordingly, this type of arrangement requires a polarity distinction between pacing and sensing pulses. Furthermore, the placement of the sensor is taught to be in the atrium and there is a potential that oxygen monitoring may be compromised if the sensor is located immediately adjacent to or in contact with the atrium wall because there may be an insufficient amount of blood in the area surrounding the sensor to obtain a reliable measure of oxygen concentration.